1. Field of the Invention
The present invention relates to a method for producing a carbonate, in which the carbonate offering orientation birefringence and being formed from needle-shaped or rod-shaped particles can be efficiently provided, and in which the particle size of the carbonate can be controlled.
2. Description of the Related Art
Conventionally, carbonates (e.g. strontium carbonate) have been widely used in the fields of rubber, plastic, paper and the like. In recent years, high-performance carbonates have been developed one after another and widely used for many purposes according to their specific features such as particle shape and particle diameter.
Examples of the crystalline forms of carbonate include calcite, aragonite and vaterite. Amongst these, aragonite is useful in a variety of applications because it is formed from needle-shaped particles and is therefore superior in strength and elastic modulus.
As methods for producing carbonates, a method in which a carbonate ion-containing solution is made to react with a chloride-containing solution to produce a carbonate and a method in which a chloride is made to react with carbonic acid gas to produce a carbonate are generally known, for example. As methods for producing needle-shaped carbonates with an aragonite structure, the following methods have been proposed, for example: a method in which a carbonate ion-containing solution is made to react with a chloride-containing solution while an ultrasonic wave is applied in the aforementioned method (refer to Japanese Patent Application Laid-Open (JP-A) No. 59-203728), a method in which needle-shaped aragonite crystals as seed crystals are previously added into an aqueous slurry of Ca(OH)2 and the seed crystals are grown only in a predetermined direction in a process of introducing carbon dioxide into the aqueous slurry of Ca(OH)2 (refer to U.S. Pat. No. 5,164,172), and a method in which sodium aluminate is added to a calcium hydroxide slurry, then the mixture is heated to a temperature of 50° C. or higher, and carbonic acid gas is blown onto the mixture (refer to JP-A No. 08-2914).
However, the method for producing a carbonate disclosed in JP-A No. 59-203728 presents a problem in which not only is the obtained carbonate formed of large particles of 30 μm to 60 μm in length but also the particle size distribution is wide, thereby making it impossible to obtain a carbonate having a desired, controlled particle size. Also, the method for producing a carbonate disclosed in U.S. Pat. No. 5,164,172 merely makes it possible to obtain large particles of 20 μm to 30 μm in length. Meanwhile, the method for producing a carbonate disclosed in JP-A No. 08-2914 requires that heating be controlled in a production step.
Incidentally, there has been a strong tendency in recent years that polymer resins are used as materials for general optical components (e.g. lenses for eyeglasses, and transparent plates) and optical components designed for optoelectronics, particularly optical components used in laser-related devices such as optical disc devices for recording sound, pictures, textual information, etc. One of the reasons for this is that optical polymer materials (optical materials made of polymer resin) are generally light, inexpensive and excellent in formability and productivity, as compared with other optical materials (e.g. optical glasses). In addition, polymer resins are advantageous in that molding techniques, such as injection molding and extrusion molding, can be readily applied.
However, it has been known that when conventional optical polymer materials are subjected to any of the molding techniques for the purpose of commercialization, the obtained products exhibits birefringence. Although birefringent optical polymer materials are not particularly problematic when used in optical elements which do not require very high optical precision, there has been an increased demand in recent years for higher-precision optical articles. For example, birefringence causes a serious problem in rewritable magneto optical discs. Specifically, since any such magneto optical disc utilizes a polarized beam as a reading or writing beam, the presence of a birefringent optical element (e.g. the disk itself or a lens) in an optical path has a negative effect on precision of reading or writing.
Accordingly, a non-birefringent optical resin material formed from a polymer resin and fine inorganic particles, the birefringence values of which are opposed to each other in sign, has been proposed for the purpose of reducing the degree of birefringence (refer to International Publication No. WO 01/25364). The optical resin material is prepared by a process referred to as “crystal doping”. Specifically, a large number of fine inorganic particles are dispersed in polymer resin, a molding force is externally applied by means of drawing or the like, allowing linking chains present in the polymer resin and the fine inorganic particles to align substantially parallel to each other, and the birefringence brought about by the alignment of the linking chains of the polymer resin is offset by the birefringence of the fine inorganic particles that has a value of the opposite sign.
As described above, in order to obtain a non-birefringent optical resin material by means of crystal doping, fine inorganic particles which are available for the crystal doping are essential. Here, it is recognized that fine needle-shaped or rod-shaped carbonate particles can be particularly suitably used as these fine inorganic particles.
Against the background of these circumstances, a number of approaches have been taken to control sizes and forms of the fine inorganic particles; however, a problem of large variance of particle sizes remains unsolved. Moreover, in conventional synthesis methods, obtained particles have poor crystallinity, and thus it is feared that optical properties (birefringence of particles) may be degraded.
To solve these problems, a method is described in Langmuir, 2005, vol. 21(1), pp. 100 to 108, in which nanosized seed crystals (calcium carbonate) are produced using PAA (polyacrylic acid), and calcium carbonate particles are obtained using these nanosized seed crystals. The study indicates a fact that in some cases a crystal type of calcium carbonate (there are three crystal types as to calcium carbonate, namely calcite, vaterite, and aragonite) can be controlled by selecting seed crystals of the desired crystal type; however, obtained particles are mostly shaped like spindles, spheres or the like, and it should be particularly noted that particles having a large aspect ratio have not yet been obtained.
Meanwhile, Chem Mater. 2003, vol. 15(6), pp. 1,322 to 1,326 describes a method in which carbonate particles are formed by a decomposition reaction of urea (thermal decomposition or enzymatic decomposition using urease) in a solution containing strontium salt or barium salt and urea. In terms of the decomposition rate of urea, enzymatic decomposition proceeds faster than thermal decomposition and is preferable; however, thermal decomposition is preferable in some cases in terms of reduction of impurities.
Since the thermal decomposition of urea proceeds slowly, numbers of cores produced become small. Although needle-shaped particles having a large aspect ratio can be obtained, the particles are large in size (see FIG. 1c in this literature). Meanwhile, in the case where urea decomposition is conducted using an enzyme, particles are formed into the shape of spheres, and the enzyme (protein) is contained in the particles, so that when such particles are introduced into a final material, there arises a concern over contamination, etc.